1. Field of the Invention
The present invention generally relates to a control system of an optical pickup to be used for an optical disc reproducing apparatus, and more particularly relates to a tilt servo system to cope with a tilt of an optical disc.
2. Description of the Related Art
A DVD is an optical disc whose diameter is 12 cm! similarly to a CD (Compact Disc). However, in the recording density, the DVD is 6 to 8 times as high as the CD. Therefore, the DVD has the ability to record a large amount of digital information, such as animation, information using in a computer or the like, thereon. Thus, the DVD is recently attracted as a digital recording medium having a large amount of recording capacity.
Further, in order to increase the recording capacity, a laser light source used for the DVD has a wavelength of 650 nm!. In contrast, a laser light source used for the CD has a wavelength of 780 nm!. Thus, the wavelength of the laser light source used for the DVD is shorter than that of the laser light source used for the CD. Further, an objective lens used for the DVD has a numerical aperture of 0.6. In contrast, an objective lens used for the CD has a numerical aperture 0.45. Thus, the numerical aperture of the objective lens used for the DVD is greater than that of the objective lens used for the CD. Furthermore, in order to increase the recording capacity, an MPEG2, which is a kind of a data compression algorithm, is used for the DVD. Therefore, the digital information of 5 Gbyte can be recorded on one side of the DVD. Thus, the high density recording is realized by the DVD. Furthermore, in order to reproduce the digital information recorded on the optical disc in the high density, a diameter of a spot of a laser beam used for the DVD is very small compared with the CD.
As mentioned above, in the reproducing apparatus used for the DVD, the wavelength of the laser light source is relatively short and a numerical aperture is relatively large. Therefore, a wave aberration is easily occurred when a part of the optical disc is slightly curved or leaned, and a margin of a tilt angle is reduced. Therefore, it is difficult to prevent the occurrence of the wave aberration. In addition, the tilt angle means a difference between a plane of the optical disc and a plane perpendicular to an optical axis of an optical pickup.
In order to compensate the wave aberration, it is attempted to use a liquid crystal panel in an optical disc reproducing apparatus. Namely, the liquid crystal panel is positioned on an optical path of a laser beam radiated from a laser light source to an optical disc. Further, a tilt sensor detects a tilt of the optical disc as a tilt error, and outputs a control voltage corresponding to the tilt error. The liquid crystal of the liquid crystal panel produces a phase difference in the laser beam on the basis of the control voltage when the laser beam goes through the liquid crystal panel. Thus, the wave aberration can be compensated.
However, a liquid crystal generally has a response delay. The liquid crystal does not perform at the same time that a control voltage is supplied thereto. Namely, the liquid crystal performs after the elapse of the response delay time. Thus, a tilt error delayed for the response delay time is used for the compensation of the wave aberration. That is to say, the past tilt error is used for the compensation of the wave aberration. If the change of the tilt error is sufficiently slow compared to the response delay time, the response delay does not affect, so that the wave aberration can be compensate correctly. However, the change of the tilt error is fast compared to the response delay time, the response of the liquid crystal panel cannot follow the change of the tilt error. Therefore, the wave aberration cannot compensated correctly.